This invention relates to a split cryostat superconducting magnet having a cryogen reserve to extend the superconducting ride-through period.
As is well known, a superconducting magnet can be made superconducting by placing it in an extremely cold environment, such as by enclosing it in a cryostat or pressure vessel containing liquid helium or other cryogen. The extreme cold ensures that the magnet coils can be made superconducting, such that when a power source is initially connected to the coil (for a relatively short period) current continues to flow through the coils even after power is removed due to the absence of resistance, thereby maintaining a strong magnetic field. Superconducting magnets find wide application in the field of Magnetic Resonance Imaging (hereinafter MRI).
Another problem encountered by conventional and early MRI equipments is that they utilize solenoidal magnets enclosed in cylindrical structures with a central bore opening for patient access. However, in such an arrangement, the patient is practically enclosed in the warm bore, which can induce claustrophobia in some patients. The desirability of an open architecture magnet in which the patient is not essentially totally enclosed has long been recognized. Unfortunately, an open architecture structure poses a number of technical problems and challenges.
One type of open architecture superconducting magnet utilizes a split dewar or split liquid helium vessels with the lower helium vessel and the upper helium vessel connected by a helium passageway or transfer to be to enable gravity flow of helium between the split helium vessels. A single helium recondenser is connected to the upper helium vessel to receive the boiled helium gas from both vessels for recondensing back to liquid helium which is flowed into the upper helium vessel and by gravity through the vertical transfer tube to the lower helium vessel. A loss of sufficient liquid helium in either vessel can cause highly undesirable quenching or discontinuance of superconducting operation of the magnet. Replenshing the liquid helium supply followed by restarting superconducting operation is expensive in terms of cost and down time of the MRI equipment. Such a loss of liquid helium can result, for example, from failure of the mechanical cryocooler associated with the helium recondenser. The cryocooler is typically positioned in a sleeve which enables cryocooler replacement without opening the helium vessels to the outside. However, replacement of the cryocooler must be made in the period after the problem is detected and before superconducting operation ceases. This period is known as the ride-through period during which the final period of superconducting magnet operation continues before quenching of the magnet.
It is highly desirable to be able to extend the ride-through period to provide sufficient time for detection and correction of the problem such as by replacement of the cryocooler.